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Old East Norse is in Sweden called Runic Swedish and in Denmark Runic Danish, but until the 12th century, the dialect was the same in the two countries. The dialects are called runic because the main body of text appears in the runic alphabet. Unlike Proto-Norse, which was written with the Elder Futhark alphabet, Old Norse was written with the Younger Futhark alphabet, which only had 16 letters. Due to the limited number of runes, some runes were used for a range of phonemes, such as the rune for the vowelu which was also used for the vowels o, ø and y, and the rune for i which was also used for e.

A change that separated Old East Norse (Runic Swedish/Danish) from Old West Norse was the change of the diphthongæi (Old West Norse ei) to the monophthonge, as in stæin to sten. This is reflected in runic inscriptions where the older read stain and the later stin. There was also a change of au as in dauðr into ø as in døðr. This change is shown in runic inscriptions as a change from tauþr into tuþr. Moreover, the øy (Old West Norse ey) diphthong changed into ø as well, as in the Old Norse word for "island".

From 1100 and onwards, the dialect of Denmark began to diverge from that of Sweden. The innovations spread unevenly from Denmark which created a series of minor dialectal boundaries, isoglosses, ranging from Zealand to Svealand.